This invention relates generally to the field of hard disc drive data storage devices, and more particularly, but not by way of limitation, to latch mechanisms for holding an actuator mechanism of a disc drive at a park position in the absence of power.
Disc drives of the type known as xe2x80x9cWinchesterxe2x80x9d disc drives, or hard disc drives, are well known in the industry. Such disc drives magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 15,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative pneumatic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspension tabs.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator bearing housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator bearing housing opposite to the coil, the actuator bearing housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator bearing housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator bearing housing rotates, the heads are moved radially across the data tracks along an arcuate path.
The movement of the heads across the disc surfaces in disc drives utilizing voice coil actuator systems is typically under the control of closed loop servo systems. In a closed loop servo system, specific data patterns used to define the location of the heads relative to the disc surface arc prerecorded on the discs during the disc drive manufacturing process. The servo system reads the previously recorded servo information from the servo portion of the discs, compares the actual position of the actuator over the disc surface to a desired position and generates a position error signal (PES) reflective of the difference between the actual and desired positions. The servo system then generates a position correction signal which is used to select the polarity and amplitude of current applied to the coil of the voice coil actuator to bring the actuator to the desired position. When the actuator is at the desired position, no PES is generated, and no current is applied to the coil. Any subsequent tendency of the actuator to move from the desired position is countered by the detection of a position error, and the generation of the appropriate position correction signal to the coil.
When power to the disc drive is lost, servo control of the current flow in the coil of the voice coil actuator is lost. In the absence of DC current flowing in the coil, the actuator is free to move in response to such things as mechanical shock, air movement within the disc drive or mechanical bias applied to the actuator by the printed circuit cable (PCC) used to carry signals to the coil and to and from the heads mounted on the actuator. Since a power loss also means that the spindle motor will also cease to rotate the discs, the air bearing supporting the heads also begins to deteriorate and contact will be made between the heads and the discs. Because of this, it is common practice in the industry to monitor input power to the disc drive, and, at the detection of power loss, to drive the actuator to a park position and latch it there until power to the disc drive is restored. One way in which the heads may be held at positions spaced from the discs is by providing ramps at the outer diameters of the discs. The ramps guide the heads away from the discs by forcing biased load suspension tabs away from the planes of the discs as the actuator moves into its park position. Such an arrangement can be seen in FIG. 2.
Once the actuator is in the park position, it is common to provide a primary latch mechanism which serves to prevent the actuator from being moved out of the park position when the drive is subjected to shock. Many forms of such latches have been used and are disclosed in the art. Examples are inertial latches and air latches. Inertial latches move in response to external shocks to lock the actuator in place. An air latch holds the actuator in place when the actuator reaches its park position, but moves to an unlatch position in response to airflow generated by the spinning discs when power is restored to the drive. One problem associated with these latches is that under certain conditions they do not effectively prevent movement of the actuator. For example, while high levels of shock will cause an inertial latch to move to engage the actuator, lower levels of shock, especially repeated shocks resulting from vibration, can be insufficient to move the inertial latch but sufficient to move the actuator out of the park position. In the case of an air latch, external shocks can move the latch out of engagement with the actuator, leaving the actuator free to rotate out of the park position. For this reason, it is known to provide a secondary latch mechanism which prevents the actuator from leaving the park position even under circumstances when the primary latch mechanism is ineffective.
This secondary latch mechanism is known to take the form of a detent in the surface of the ramp. This is illustrated in FIG. 3 which shows a load suspension tab 136 in three positions: seated in the detent 154 at left, ascending or descending the sloped ramp surface 152 in the center, and flying above the surface of the disc 110 at right. As may be seen in FIG. 4, the detent 154 increases the torque required to drive the actuator out of the park position against the bias of the load suspension tabs 136, so the actuator will remain parked even when the primary latch is ineffective. However, there are many disadvantages associated with the use of a detent as a secondary latch mechanism, as will be explained below.
One disadvantage of these detent latch designs is that the torque required to move the actuator out of the park position decreases when a disc drive is depopulated, i.e., when discs are removed from the drive. Because heads and corresponding load suspension tabs are also removed, the overall bias provided by the load suspension tabs against the ramps is reduced, thereby decreasing the overall secondary latch torque. Shocks to the disc drive which are too low to activate an inertial latch but high enough to cause the actuator to overcome the reduced secondary latch torque could allow the actuator to leave the park position and contact the stationary disc, causing damage to the discs and heads.
Another problem presented by these detent latch designs is that the detent depth permits excess vertical acceleration of the heads when the drive is subject to high levels of shock. As can be seen in FIG. 9, detents 154 which face each other define a large distance 240 over which a head may travel when the drive is subjected to shock, resulting in greater acceleration and therefore velocity, increasing the likelihood of damage when the tab 136 contacts the opposing detent.
Another problem with these detent latch designs is that they are by nature limited in both xe2x80x9clatch torque,xe2x80x9d i.e., the force by which they prevent movement of the actuator out of its park position, and xe2x80x9crange of influence,xe2x80x9d i.e., the range of actuator rotation over which they exert torque on the actuator. Referring to FIG. 9, latch torque is increased by increasing the angle 210, such that increasing amounts of torque are required to drive the actuator against the bias force provided by the load suspensions tabs 136 against the detents 154. The latch torque is limited because if the angle 210 is increased too much, the load suspension tabs 136 can be bent or broken when forced against the detent surface. The range of influence 230 of the detent latch is limited because the detent depth 220 is limited by the spacing of adjacent heads from one another. As can be seen in FIG. 9, if the depths 220 of the detents 154 were increased too much, the heads supported by the suspension load tabs 136 would collide with one another when the actuator reached its park position. If latch torque is to be maintained at a suitable level, angle 210 must also be maintained and the range of influence 230 is limited by these factors.
A related problem with these detent latch designs is that the latch torque cannot be varied independently of the range of influence. If the latch torque is increased, for example, by increasing the angle 210 of the detent 154, the range 230 over which the detent latch is effective is necessarily shortened because of detent depth limit 220.
The limited range of influence of these detent latch designs can also affect the structure of the primary latch mechanism when an inertial latch is used. When manufacturing tolerances are taken into account, a one-piece actuator may rotate as little as 1.5 degrees before escaping the detent latch. When the drive is subject to shock under these conditions, the actuator may rotate past its point of engagement with the inertial latch before the inertial latch can move to its latching position, thereby allowing the actuator to descend the ramp and contact the disc surface. A solution to this problem in the past involved constructing a two-part latch in which an xe2x80x9cengagementxe2x80x9d part of the latch was moved more quickly by an xe2x80x9cinertialxe2x80x9d part to compensate for the slow movement of the xe2x80x9cinertialxe2x80x9d portion. However, this inertial latch structure involves additional cost associated with additional manufacturing steps, tooling, and packaging space in the VCM area of the drive.
The present invention is an improved secondary latch mechanism for holding a disc drive actuator in its park position. The secondary latch mechanism is a non-contact magnetic latch which includes a magnetically permeable element coupled to the actuator. The magnetically permeable element is positioned so as to be attracted to the magnetic field produced by the voice coil magnets when the actuator is in or near its park position. Additional features and benefits will become apparent upon a review of the attached figures and the accompanying description.